This application is based on and claims the priority under 35 U.S.C. xc2xa7119 of German Patent Application 199 48 819.3, filed on Oct. 9, 1999, the entire disclosure of which is incorporated herein by reference.
The invention relates to a heater arrangement such as a heater cable including a connector and/or a terminating element at an end thereof, and using a fluoropolymer material to provide a seal. The invention further relates to a method of manufacturing such a heater arrangement.
Heater arrangements of the above mentioned general type are used for heating pipe systems such as water lines, water tanks, valves and armatures as well as structural components in which liquid products are to be transported, in order to achieve an active frost protection to prevent freezing of the liquid being transported therein when the surrounding ambient environment drops to temperatures below the freezing temperature. Especially in aircraft, which are subjected to extremely low outside temperatures during long flights, it is possible that the various water lines including fresh water lines and waste water lines will freeze if an adequate frost protection is not provided. This could lead to the failure of the entire fresh water or waste water system of the aircraft and, for example, make it impossible for the passengers to use the toilets or the like. For this reason, it has become commonly known to use such heater arrangements, especially in the form of heating bands or heating cables, on the water systems of aircraft.
The heating bands that are used for such heater arrangements conventionally comprise a heating element as well as connecting conductors or conductor strands. The heating element itself is insulated and protected by an insulating layer of a fluoropolymer synthetic plastic. Onto this inner jacket of fluoropolymer, a protective shielding conductor braid is arranged, and then a plastic outer protective jacket or sheath encloses this heater arrangement. The heating bands are typically commercially available as xe2x80x9cyard goodsxe2x80x9d. In other words, the heating band is available in the form of a long continuous coil, and an appropriate length of the heating band is simply cut from the coil and must then be assembled or fabricated with other components to meet the need of the particular application, as follows.
A connecting member must be provided on at least one end of the heating band in order to enable the interconnection of several heater arrangements or the connection of a respective heater arrangement to a source of electrical power. In order to achieve this, an end of the heating band is xe2x80x9cstrippedxe2x80x9d to remove the protective jackets or sheaths on the inside and the outside, so as to expose the connection conductors. The heating element portion itself is shortened so that the connection conductors are sufficiently exposed to reach an adequate length so as to then be used as an interconnection line. The individual strands of the protective conductor braid must be combed out and then twisted together, and then connected to an additional extension line. The respective lengths of the connection conductors and of the extension line of the protective braid are matched or adapted to each other so that they respectively end flush with one another.
The transition region between the heating band and the connection lines or conductors must be sealed in a pressure-tight manner that is also resistant to chemical influences and substantial temperature variations, which requires a rather complicated and time consuming process. This is necessary if the heater arrangement is to be used in an aircraft, because for such applications, the influences of vibrations, greatly varying temperatures, low pressure conditions, and the influence of various aggressive chemical agents must necessarily be taken into account so that it can be ensured that the heater arrangement does not fail due to such influences.
In the above context, to achieve such a sealing of the transition region at the connection end of a heater band, the prior art calls for a process in which at least respectively one adhesive and/or potting mass is manually applied to each connection area and then covered with a shrinkable tubing such as a heat shrink tube. Moreover, a complex adhesive pretreatment is necessary, whereby the following steps must be carried out in order to achieve a sufficient bonding between the epoxy adhesive and the fluoropolymer plastic surface (e.g. fluorinated ethylene propylene-FEP) of the heater band: the outer and inner jackets or sheaths must be abraded or otherwise roughened; the region to be adhesively bonded must be cleaned with a solvent or cold cleaning agent, for example ethanol; an etching compound, for example TETRA-ETCH, must be applied to the area and allowed to take effect; then the etchant must be washed away using de-ionized water and subsequent cleaning with a solvent or cold cleaning agent; then the heating band must be dried in a circulating air oven; and finally after the above pretreatment, the epoxy adhesive must be applied and the adhesive bonding of the shrinkable tube must be carried out within a limited period of time.
Since several shrinkable tubes are adhesively bonded in several layers, the time and complexity involved in the above fabrication process for fabricating this connection area is multiplied accordingly. Moreover, when the finished connection area is subjected to mechanical loads, for example during installation or due to operational vibrations, it has been found that the adhesively bonded joints can become separated, especially because the cured epoxy adhesive is relatively rigid while the fluoropolymer plastic such as FEP remains relatively flexible, so that the two materials are not very compatible with each other in relation to mechanical flexure.
A further difficulty or disadvantage with the conventional process for fabricating the connection areas as described above is that a specialized workplace with its own air exhaust or air processing system is necessary for carrying out at least the etching step of the process. Namely, the use of such etching compounds raises environmental concerns as well as health and safety concerns for the workers carrying out the above process. Finally, the disposal or reprocessing of the etching compounds involves additional costs and difficulties.
The above processes are also necessary for fabricating a terminating member on a terminal end of such a heater arrangement. Namely, for such a terminating member it is similarly necessary to carry out an etching process to prepare the components for a subsequent adhesive bonding using an epoxy adhesive on the FEP material of the protective jacket or sheath.
U.S. Pat. 5,998,772 (Kirma et al.) and U.S. Pat. 6,126,483 (Kirma et al.), which are both commonly owned with the present application, disclose heating conductor arrangements that can be coupled to one another or connected to an appropriate electrical power supply by means of special connection modules. Generally, these patents call for a connection technology in which the transition or joint area is provided with a split housing, whereby the interior space of the housing is completely filled with an adhesive and potting compound after completion of the necessary connections, in order to provide a reliable protection against the penetration of moisture or liquid. In the event that fluoropolymer (e.g. FEP) sheaths or jackets are used, in order to achieve a good adhesion of the potting compound with the FEP surfaces, the disclosed systems and processes would also require an adhesive pretreatment, for example using an etching technique. Thus, the above mentioned disadvantages such as a time intensive and complicated pretreatment process, health and safety risks while using various etching compounds such as TETRA-ETCH, as well as the necessary disposal or reprocessing of the etching compounds, will also be expected to apply in the context of these patented technologies.
The present invention is related to and provides a further improvement over the technologies disclosed in the above mentioned U.S. Pat. Nos. 5,998,772 and 6,126,483. The entire disclosure of each of these prior U.S. Patents is incorporated herein by reference to provide background information.
The general problem in the conventional techniques discussed above is that the fluoropolymer surfaces of the jackets or sheaths of the heating conductors and the like, which are made of fluorinated ethylene propylene (FEP) for example, are not compatible with and do not readily establish a good bond with respect to epoxy adhesives or other typical adhesive and potting materials. The prior art has not yet found a solution to this problem, and so it has not been possible to achieve an adequate seal in an acceptably simple manner when using FEP or the like as the material of the jackets of the components of the heater arrangement.
In view of the above, it is an object of the invention to provide a heater arrangement of the general type described above, which has at least one connecting element and/or at least one terminating element, which is especially suited for use as a heater for aircraft water systems, and achieves an adequate seal to prevent the penetration of moisture and liquids into the connection region of the heater arrangement, while resisting the influence of vibrations, substantial temperature variations, low pressure conditions, and various aggressive chemicals. It is another object of the invention to provide a method of assembling or fabricating such a heater arrangement that is simpler, less time consuming, more economical, and safer than the prior art methods. It is a further particular object of the invention to retain the benefits of using a fluoropolymer material for the sheath or jacket of heater arrangement components, while nonetheless easily achieving a reliable seal relative to the fluoropolymer material. The invention further aims to avoid or overcome the disadvantages of the prior art, and to achieve additional advantages, as apparent from the present specification.
The above objects have been achieved according to the invention in a heater arrangement comprising a heating element and a connecting element and/or a terminating element connected to the heating element. Throughout this specification, the connecting element will also be referred to as a xe2x80x9cconnectorxe2x80x9d, and the terminating element will also be referred to as a xe2x80x9cterminatorxe2x80x9d. The heating element in turn comprises a heating band and heating conductors, and is further enclosed or ensheathed by a fluoropolymer protective layer. Furthermore, especially according to the invention, an additional fluoropolymer synthetic material is provided in the area of the connector and/or the terminator in order to achieve a seal in this area. For example, the heater arrangement may further comprise a molded plastic part, preferably made of FEP, arranged in the area of the connector or the terminator to provide a transition and contribute to the seal between the end of the heating band and the connector or the terminator. A further fluoropolymer layer may be thermally welded, melted, fused or extruded to the molded plastic part in a transition area. In the case of the heater arrangement including a terminating element or terminator, the molded plastic part may be in the form of an end cap that closes and insulates the terminal end of the heating band.
The above objects have further been achieved according to the invention in a method of fabricating a connector element or a connector on an end of a heater arrangement, including the following steps. A heating band with at least one heating band conductor is cut open so as to expose the at least one heating band conductor in a prescribed area based on prescribed length dimensions needed for a particular application, in order to fabricate at least one connector at the cut location. By means of an extrusion process, a fluoropolymer synthetic plastic layer, preferably consisting of fluorinated ethylene propylene (FEP), is applied onto the heating band. A shielding braid is arranged on the fluoropolymer synthetic plastic layer. The shielding braid is cut back in the area of at least one connector, whereby respectively a connection or junction lead is formed at the cut location of the respective shielding braid. One end of a respective extension line is connected to the respective junction lead. At least one further extrusion process is carried out to apply an outer protective jacket or sheath of a fluoropolymer synthetic plastic, preferably FEP, onto the heater arrangement and at least over partial areas of the connecting elements. The areas of the connecting elements are then completed by connecting power lines or the like to the heating band conductors, and/or by providing contact elements on the heating band conductors and/or the extension line and/or the power lines.
A variant of the method according to the invention relates to fabricating a terminating element or terminator on a terminal end of a heater arrangement, comprising the following steps. A heating band having at least one heating conductor is cut open so as to expose the at least one heating conductor at a prescribed area based on a prescribed length dimension, for receiving at least one terminating element at the cut area. By means of an extrusion process, a fluoropolymer synthetic plastic layer, preferably consisting of FEP, is applied onto the heating band. A protective braid is arranged on the fluoropolymer synthetic plastic layer and then cut back in the area of the terminating element. The strands of the heating conductors are freed or exposed in the end region of the terminating element. By means of a further extrusion process, an outer protective jacket or sheath of a fluoropolymer plastic, preferably FEP, is applied onto the heater arrangement and at least partially or on partial areas on the terminating element areas of the heater arrangement. Then, to achieve a final seal, i.e. to achieve a sufficient sealing, the FEP material in the end portion of the terminating element or elements is thermally welded or extruded.
It is especially advantageous that the invention avoids surface treatments of the FEP layer, which were necessary according to the prior art, and which are potentially dangerous or injurious to the health of the workers carrying out such treatments, and also avoids the use of environmentally undesirable agents such as etching agents. Also, the inventive arrangement omits any further plastic materials in addition to the fluoropolymer synthetic plastic material FEP, which exhibits the best possible characteristics of resistance to the SKYDROL hydraulic oil typically used in aircraft hydraulic systems, moisture resistance and protection, good insulating protection, as well as a good flexibility. Thus, the invention maintains the advantageous use of FEP as a jacketing or sheathing material, while avoiding the previously associated with the difficulty of forming a seal relative to the FEP material. Also according to the invention, additional components, such as shrinkable tubes or the like are no longer necessary, and thereby the time consuming and complex processes for fabricating the connecting element and/or the terminating element on an end of the heater arrangement have been avoided or significantly simplified.